1. Field of the Invention
This invention relates to a magnetic recording medium and a process for the production thereof and more particularly, it is concerned with a magnetic recording medium with a low noise level and a process for the production of the same.
2. Description of the Prior Art
Ferromagnetic materials used in magnetic recording substances such as audio tapes, video tapes, memory tapes, magnetic sheets and magnetic cards are fine powders of ferromagnetic iron oxides, cobalt ferrite, ferromagnetic chromium dioxide and ferromagnetic metals or thin films of ferromagnetic metals. These magnetic recording substances have been used in a wide variety of technical fields wherein electric or magnetic signals are recorded and reproduced and, of late, a system of recording, in particular, a short wave-length signal in a high density has been watched with keen interest. Accordingly, magnetic recording properties suitable for high density recording, for example, a considerably high coercive force and large residual flux density are required for ferromagnetic materials. Moreover, it is necessary, for example, for magnetic cards, that demagnetization due to heating or pressing is little. Metallic ferromagnetic materials have been considered most promising for high density recording with low demagnetization.
On the other hand, the recording wave-length in a video tape is much shorter than in a sound recording. For example, in a VTR (video tape recorder) for broadcasting, a short wave to a minimum wave-length of about 2 microns has to be recorded. In particular, a VTR using a minimum wave-length of about 0.6 to 1 micron has lately been developed. However, the above described oxide-type magnetic substances are not suitable for magnetic recording of a signal of short recording wave-length (about 2 microns or less), because their magnetic properties such as particle shape, particle size, coercive force and residual magnetic flux density are insufficient for high density recording.
Development of ferromagnetic metal powders capable of satisfying these properties and suitable for high density recording has lately been carried out actively. The following six methods are known as a method of preparing the ferromagnetic metal powder:
(1) A method comprising heat-decomposing an organic acid salt of a ferromagnetic metal and reducing with a reducing gas. This method is described in, for example, Japanese Pat. Publication Nos. 11412/1961; 22230/1961; 14809/1963; 3807/1964; 8026/1965; 8027/1965; 15167/1965; 16899/1965 (U.S. Pat. No. 3,186,829); 12096/1966; 14818/1966 (U.S. Pat. No. 3,190,748); 24032/1967; 3221/1968; 22394/1968; 29268/1968; 4471/1969; 27942/1969; 38755/1971; 38417/1971; 41158/1972; and 29280.
(2) A method comprising reducing a needle-like iron oxyhydroxide, substance containing metals other than iron as well as the above oxyhydroxide or needle-like iron oxide derived from the oxyhydroxide. This method is described in, for example, Japanese Patent Publication Nos. 3862/1960; 11520/1962; 20335/1964; 20939/1964; 24833/1971; 29706/1962; 30477/1972 (U.S. Pat. No. 3,598,563); 39477/1972 and 24952/1973; Japanese patent application (OPI) Nos. 5057/1971; 7153/1971; 79153/1973; and 82695/1973 and U.S. Pat. Nos. 3,607,220 and 3,702,270.
(3) A method comprising evaporating a ferromagnetic metal in an inert gas at a low pressure. This method is described in, for example, Japanese Pat. Publication Nos. 25620/1971; 4131/1972 and 27718/1972 and Japanese patent application (OPI) Nos. 25662/1973; 25663/1973; 25664/1973; 25665/1973; 31166/1973; 55400/1973 and 81092/1973.
(4) A method comprising heat-decomposing a metal carbonyl compound. This method is described in Japanese Patent Publication Nos. 1004/1964; 3415/1965 and 16868/1970; and U.S. Pat. Nos. 2,983,997; 3,172,776; 3,200,007 and 3,228,882.
(5) A method comprising electrodepositing a ferromagnetic metal powder using a mercury cathode and then separating the metal powder from mercury. This method is described in, for example, Japanese Pat. Publication Nos. 12910/1960; 3860.1961; 5513/1961; 787/1964; 15525/1964; 8123/1965; 9605/1965 (U.S. Pat. No. 3,198,717); 19661/1970 (U.S. Pat. No. 3,156,650) and U.S. Pat. No. 3,262,812.
(6) A method comprising reducing a solution containing a ferromagnetic metal salt by adding a reducing agent thereto. This method is described in, for example, Japanese Pat. Publication Nos. 20520/1963; 26555/1963; 20116/1968; 9869/1970; 14934/1970; 7820/1972; 16052/1972; 41718/1972; 41719/1972 (U.S. Pat. No. 3,607,218); Japanese patent application (OPI) Nos. 1353/1972 (U.S. Pat. No. 3,756,866), 1363/1972; 42252/1972; 42253/1972; 44194/1973; 79754/1973; 82396/1973 and 41899/1974 and U.S. Pat. Nos. 3,206,338; 3,494,760; 3,535,104; 3,567,525; 3,661,556; 3,663,318; 3,669,643, 3,672,867; and 3,726,664.
In a tape wherein a ferromagnetic metal powder obtained by each of these methods is mixed or dispersed in a binder and dispersing agent and coated onto a support member, however, the noise level is high due to that the dispersibility of the ferromagnetic metal powder is low, which constitutes a bar for the practical use of a magnetic recording medium using such a ferromagnetic metal powder. This is considered to be due to the fact that the ferromagnetic metal powder has an active surface and hydrophilic property and is hardly miscible with the binder. In particular, a ferromagnetic metal powder obtained by the reduction with a phosphinate compound or borohydride compound in aqueous solution has a very low miscibility with binders and, therefore, it is difficult to prepare a magnetic recording medium with a high output and low noise level from such a ferromagnetic metal powder.